If you already have two IB woofers and you're far from satisfied then I realize that you will expect the new addition to do the bulk of the work, so it better be something very powerful.
In an ideal situation I would prefer two of the JL marine sealed woofers (excluded because of mounting surface area) or the JL 10W6 in a bass-reflex enclosure (excluded because of enclosure displacement). Either of these would provide alot of radiating piston area and a high degree of responsiveness. Keep in mind that a bass-reflex enclosure can have the same impact as doubling your radiating area and doubling the sensitivity.
Between the 10W6 and 13TW5 in a sealed enclosure, I think that is an easy choice. The 13.5-inch 13TW5 has 93 percent more piston area, is greater than 2dB more sensitve and will use more of your available enclosure displacement (providing a lower resonance once put to task). Btw, this slimline speaker has a 7-inch voice coil and demonstrates none of the typical shortcomings of other slimline woofers. Sound quality is excellent in every respect.
I like the fact that it has a butyl rubber surround for direct UV exposure. One sidenote though, this woofer demands that you have an absolutely airtight enclosure if you want it to live up to its full potential. No compromise in this case. This means that when you press on the woofer it should not move at. Easily stated but a little harder to execute.